windows-nat.c (windows_get_absolute_argv0): New function.
windows-nat.h: Add its prototype.
main.c (get_init_files): Use filename_ncmp instead of strncmp.
Use IS_DIR_SEPARATOR instead of looking for a character inside
SLASH_STRING. Include filenames.h.
(captured_main) [__MINGW32__]: Make argv[0] absolute, so that
relocate_gdb_directory works when passed gdb_program_name.
Include windows-nat.h.